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Joined
Mar 5, 2017
Messages
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Class: Sport
Category: Small field

I have another experimental glider in the works, this time exploring a random thought about whether a glider’s elevator could be reversed so that its leading edge rotated downward, scooping air from underneath the wing and channeling it upward instead of off its trailing edge. Whyever? Mostly curiosity, and cause I’m a fan of the unconventional and unorthodox in design.

Here's a basic sketch of what I’m planning:
shrike silhouette sketch.jpg


I have some ideas about elevator retention and how to limit the angle needed for a workable glide, but as with all experiments, I’ll just have to fabricate and see how workable it ends up being, then modify or redesign as needed.

Besides just proof of concept, a primary goal is maximizing the altitude I can get on a "B" motor by using 1/16" balsa throughout (save for the fuselage). I may be able to get away with a "C" motor by strengthening the balsa with a coat of dope (lacquer). Using a 20% to 30% diluted solution adds a surprising amount of hardness and rigidity to the balsa, as it tends to soak into the wood and reinforce it's fibers without the heavier weight penalty of paint. Of course the other benefit is that it repels humidity and water, minimizing or eliminating warpage (pretty common problem during our intensely humid summers).
 
Not an aerodynamicist, but I suspect you will find this very ineffective from a pitch control perspective. I suspect it will act more as a spoiler of lift than as a means to alter the pitch.

Some of the highest performing R/C sailplanes went through a phase where full-flying horizontal tail surfaces were all the rage (e.g. Dr. Mark Drela's Supra), but the pendulum has swung back now and the majority are using a more traditional horizontal stabilizer and hinged elevator arrangement (the latter is more effective at generating lift by changing the camber of the tail surface, rather than simply altering the angle of attack).

Watching the outcome of your experiment with interest...
 
Not an aerodynamicist, but I suspect you will find this very ineffective from a pitch control perspective. I suspect it will act more as a spoiler of lift than as a means to alter the pitch.

Some of the highest performing R/C sailplanes went through a phase where full-flying horizontal tail surfaces were all the rage (e.g. Dr. Mark Drela's Supra), but the pendulum has swung back now and the majority are using a more traditional horizontal stabilizer and hinged elevator arrangement (the latter is more effective at generating lift by changing the camber of the tail surface, rather than simply altering the angle of attack).

Watching the outcome of your experiment with interest...

I agree. it's probably not the most efficient design. My goal with this one is simply to satisfy my curiosity and have fun doing it. It's definitely not a performance-oriented planform, but I feel it's workable. And different. I like different — even odd.

I think I caught the unconventional bug after building Tim Bush's Ring Hawk (Nov/Dec 2003 Sport Rocketry magazine). Now that's something that really doesn't look like it should act like a glider post-apogee. Glides really well too, all without any airfoils or normal flight surfaces.
 
As usual I built some small test models just to see if the design would even work. The first was made of cardboard and scrap balsa, but the results were erratic, probably because the cardboard was prone to flexing too much.

test model-cardboard .jpg


The second test model was all balsa and I jury-rigged the elevator angle adjustment just using some twisted strands of thin gauge copper wire that allowed me to change the angle. Took this to the park for some test tosses. This version performs much more reliably, so I’ll move forward with further experimenting.

Obviously it’s not an ultra-performance design where glide duration is the goal, it’s definitely for sport flying in small fields. Since this test model is quite small and light, it can’t punch through wind gusts much over 8-10 mph and gets buffeted around a lot, so I’ll be working on a 126% upscale. I still haven’t decided on a mechanism to control the angle of the elevator, but for now it’ll probably be a small nylon screw adjustment like what the Estes Astron Space Plane or Skydart uses.

test model-balsa..jpg
 
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Started on the upscaled version. Went old-school using paper patterns to lay out the parts, then used a hobby knife to slice them apart. I decided to keep everything ultra-light so I’m using medium weight 1/16” balsa and reinforcing key areas.

Main center wing panel:

ctr panel.jpg


Main perimeter wings:

main wings.jpg

main wing.jpg


Fuselage:

20240410_143619.jpg
 
Although the glider is well reinforced forward and aft by the fuselage and rudders, I had a concern about strengthening it laterally to provide support and stiffness against the stresses of launch inertia, which can be considerable. So as extra insurance, I cemented in a length of bamboo skewer that I planed flat on one side to provide more contact area.

lateral reinforcement.jpg



Finished up the balance of parts to finish assembly:

20240411_151630.jpg
 
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The preliminary working CG is 3.44” from the forward apex of the wings per the online Flying wing CG calculator site.

Spectre CG.jpg
 
Minor setback: I fabricated a small flap limiter out of 1/8” balsa reinforced on one face with 1/32” ply, with a small nylon adjustment screw. Of course, mind sims don’t always turn out the way you imagined, and this one was no exception.

In this case it needs to be flush with the wing bottom surface, which I couldn’t do. That being the case, I’m going to get the best “ballpark” angle via trim tests and use a non-adjustable angle limiter. I’ll fine tune things with bits of clay weight if needed once everything is assembled.

20240416_120853.jpg

20240416_120804.jpg
 
Although I intended this to be the final glider, it’s ending up being a prototype, a beta testbed to experiment with various solutions to see what works and what doesn't. Made a few wrong turns and some improvements, and had to unglue and modify several pieces so that it’s now looking like a somewhat battered veteran.

Anyway, moving on; since there needs to be some way of limiting the elevator flap angle, I fabricated a small, simple flap limiter out of thin piano wire threaded through a 1/16” aluminum tube. Crimping one end slightly creates a friction fit and keeps the wire from sliding too freely. I’ll be able to trim the glider easily just by moving the wire forward or backward. Not the most elegant solution, but simple and easy to make, install, and adjust. (Hard to tell from the angle of the shot, but the bent portions actually lay horizontal and flat).

Flap Limiter-2.jpg



For now, elevator activation is via elastic sewing cord threaded through slots cut into the elevator flap and anchored underneath. It’ll be replaced with tan sport rubber cut to the minimum tension length to just get the elevator moving without overly stressing it. Air pressure will do the rest.

eleastic anchor.jpg
 
Completed glider received a single coat of diluted lacquer to both strengthen it and make it water/humidity-resistant to prevent warping.

trim ready.jpg

Waiting for the weather to cooperate by providing a single-digit low-wind day to allow a trimming session.
 
Had an opportunity to do some trimming this morning. Admittedly I probably underestimated the size of the elevator flap. A half inch or so would’ve made it much more effective. All I can do at this point is try to increase its span a bit by cutting the relief slot longer and fashioning a new flap.
 
I was able to remove the old flap and replace it with a slightly wider one. Guess I’ll see if it makes much of a difference the next time I get to trim it. The next one I build will definitely have at least a 5% larger elevator.
 
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Couldn’t leave it alone and tore out the replacement flap I just put in and replaced it with yet another one with more surface area. Not the neatest bit of surgery, but then again this is just the beta version. Also added some color to make it easier to locate in green crop fields or dry scrub.

20240430_165818.jpg
 
I had the chance to do a trimming session and there was a noticeable improvement as a result of the increase in flap size. Given that, I intend to increase it a bit more in the eventual final version.

Fantasy rendition of the final glider in its possible livery.

Shrike Sketch.jpg
 
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With the trimming done, I removed the temporary elevator flap limiter and replaced it with a combination control horn/elevator stop.

flap stop-control horn-1.jpg

flap stop-control horn-2.jpg


The flap is held down at a neutral angle by a kevlar cord running up to the forward end that’s captured by the motor pod hook.

glider bottom-2.JPG

cord capture-1.JPG

cord capture-2.jpg


The pin on the bottom rear of the glider is just a skid to protect the elevator flap and control horn/elevator stop. Probably overthinking it and may not need it. I’ll have to see how fast it comes down on its test flight.
 
Had an opportunity to send the Shrike up on its inaugural “proof of concept” test flight on a B4-2 and it did surprisingly well. Unfortunately it came down in waist-high scrubby underbrush and I gave it up for lost after 20 minutes of searching in stifling heat. As luck would have it, my better half found it despite the disadvantage of having her view plane more obscured by being over a head shorter.

Of note: the motor pylon needs to be reduced in height. An error in measurement placed the thrustline about 0.205” (3/16”) higher than normal and caused the glider to arc before apogee. I’ll definitely be paying better attention on the upcoming Ver 2.0 build.

 
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Started construction on Version 2.0 of the glider. This should go fairly quickly since I’ve finalized the design tweaks. What may slow progress is the usual culprit: finding suitable weather conditions to trim and test fly the glider.

shrike Ver 2.0.jpg


Shrike ver2.0-2.jpg
 
Did this ever fly? Such a cool concept!​
Thanks John! The prototype did (see video in post #17). I finished version 2.0 (this time with a lower pylon at the correct height) so it should launch much straighter. We're in the midst of a weeks-long heat wave and It's been in the high 90s and windy, plus it's hurricane season here 😬, so I'm just waiting for better flying conditions.
 
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